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Solvent effect on radical homo- and copolymerization of di-2-[2-(2-methoxyethoxy)ethoxy]ethyl itaconate
European Polymer Journal 38 (2002) 1995–2001 www.elsevier.com/locate/europolj
Solvent effect on radical homo- and copolymerization of di-2-[2-(2-methoxyethoxy)ethoxy]ethyl itaconate H. Nakamura, M. Seno, H. Naganuma, T. Sato
*
Faculty of Engineering, Department of Chemical Science and Technology, Tokushima University, Minamijosanjima 2-1, Tokushima 770-8506, Japan Received 16 November 2001; received in revised form 4 February 2002; accepted 7 February 2002
Abstract Solvent effect on homo- and copolymerization of di-2-[2-(methoxyethoxy)ethoxy]ethyl itaconate (DMEI) was studied at 50 °C using dimethyl 2,20 -azobisisobutyrate as radical initiator. The polymerization rate (Rp ) highly depended on the kind of solvent; 19 solvents were used. The highest Rp (in 1-tetradecanol) is 13 times the smallest (in chloroform). On the other hand, the solvents did not exert as great an effect on the molecular weight of the resulting polymers. The propagation rate constant (kp ) was determined in 15 different solvents by means of ESR spectroscopy. The highest kp (4.5 l/mol s in toluene) is 5.6 times the lowest (0.8 l/mol s in chloroform). A noticeable solvent effect was also observed in the copolymerization of DMEI (M1 ) and styrene (M2 ), where nine solvents were used. The highest r1 (0.46 in 1-butanol) is about 6 times the lowest (0.08 in methanol). The r2 value falls in the range of 0.2 (dimethyl sulfoxide) and 0.52 (benzene). The solvent effects thus observed were analyzed according to the linear solvation energy relationship proposed by Taft and co workers. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Itaconate; Radical polymerization; Solvent effect; ESR spectrum; Rate constant of propagation; Monomer reactivity ratio
1. Introduction Dialkyl itaconates are readily polymerized by radical initiators in spite of the severe steric requirements of their bulky a-substituents. The homogeneous polymerization systems involve ESR-observable propagating polymer radicals under actual polymerization conditions because the termination reactions are much suppressed by the substituents [1–7]. Therefore, the polymerization systems have often been used for ESR study of solvent effect [8,9], salt effect [10–12], penultimate effect [13,14], and substituent effect [3,4,15] in homogeneous radical polymerization.
Previously we found that the radical polymerization rates of dialkyl itaconates, such as di-n-butyl itaconate (DBI) and di-2-ethylhexyl itaconate (DEHI), highly depend on the kind of solvent used, and explained the large solvent effect in terms of the solvent affinity for the propagating polymer radicals [8,9]. Recently, we have observed that di-2-[2-(2-methoxyethoxy)ethoxy]ethyl itaconate (DMEI) carrying two hydrophilic oligomeric ethylene oxide side chains also experiences a significant solvent effect in the radical polymerization and copolymerization, and have studied kinetically the solvent effect by means of ESR.
*
Corresponding author. Tel.: +81-88-656-7402; fax: +81-88655-7025. E-mail address: [email protected] (T. Sato).
This paper describes the results on solvent effect during the radical polymerization and copolymerization
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H. Nakamura et al. / European Polymer Journal 38 (2002) 1995–2001
of DMEI on the basis of elementary reaction analysis by ESR.
2. Experimental 2.1. Materials DMEI was prepared by acid-catalyzed esterification of itaconic acid with 2-[2-(2-methoxyethoxy)ethoxy]ethanol using p-toluenesulfonic acid; it was purified by column chromatography using silica gel as packing and ethyl acetate as developing solvent as described previously [7]. Dimethyl 2,20 -azobisisobutyrate (MAIB) as initiator was recrystallized from methanol. Solvents were used after distillation. Commercial styrene (St) was freed from inhibitor by washing with 5% NaOH aqueous solution, dried over anhydrous Na2 SO4 , and distilled. 2.2. Measurements Gel permeation chromatography (GPC) was performed at 38 °C on a Toso HLC-802A chromotograph using tetrahydrofuran (THF) as eluent. From the GPC results, the number-average (M n ) and weight-average (M w ) molecular weights were estimated after calibration with poly(St) standards. 1 H-NMR spectra were recorded with a Hitachi R-24b (60 MHz) spectrometer. ESR spectra of the polymerization mixtures in degassed and sealed ESR tube were obtained using a JEOL JESFE2XG spectrometer at X-band (9.5 GHz) with a transverse electric wave mode cavity. The concentration of polymer radicals was determined by computer double integration of the first derivative ESR spectrum. For this, a solution of 2,2,6,6-tetramethylpiperidin-l-oxyl radical(TEMPO) in benzene was used as standard, since TEMPO was unstable in the present polymerization system. A MgO marker was used for the calibration of the radical concentration in various media. 2.3. Polymerization Polymerization and copolymerization of DMEI were carried out in degassed and sealed glass tubes with shaking at 50 °C. The resulting poly (DMEI) which contained a small amount (10%) of the monomer was isolated by pouring the polymerization mixture into a large excess of diethyl ether-cyclohexane (5:3(v/v)) mixture. Then, the conversion was corrected by GPC using a calibration curve for the relative weight between DMEI and its polymer. The conversion in the polymerization in dimethyl sulfoxide (DMSO) was determined by following the decrease in the vinyl proton signals of monomer by NMR, where the o-methylene protons of benzyl acetate were used as internal standard.
Copolymers of DMEI and St were also isolated using the diethyl ether-cyclohexane mixture as precipitant. 3. Results and discussion 3.1. Polymerization of DMEI with MAIB in various solvents DMEI was polymerized with MAIB at 50 °C in various solvents, where the concentrations of DMEI and MAIB were 1.00 mol/l and 5:00 102 mol/l, respectively. The 19 solvents used were benzene, toluene, chlorobenzene, ethyl acetate, acetone, methyl ethyl ketone (MEK), THF, chloroform, acetonitrile (ACN), diethylene glycol dimethyl ether (diglyme), N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), methanol, ethanol, 1-propanol, 1-butanol, 1-octanol, 1dodecanol, 1-tetradecanol. The polymerization proceeded homogeneously in these solvents. The polymer yield increased linearly with time without any induction period. From the time– conversion plots, the initial polymerization rate (Rp ) was estimated in each solvent. Table 1 summarizes the results, together with the molecular weights of the polymers. Rp was found to depend highly on the kind of solvent used. The highest Rp observed in 1-tetradecanol is 13 times the smallest in chloroform. On the other hand, the solvents did not exert as large an effect on the molecular weight (M n ¼ 0:7 103 –2:0 103 ) of poly(DMEI)s formed. Chain transfer to monomer is sugTable 1 Solvent effect on the polymerization of DMEI with MAIB at 50 °Ca Solvent
Rp 106 (mol/l)
M n =104
M w =104
M w =M n
Diglyme DMSO Toluene Chlorobenzene Benzene Ethyl acetate Acetone DMF THF ACN MEK Chloroform
24.1 21.5 9.0 8.9 6.8 6.2 5.8 5.8 5.6 4.4 4.2 2.8
1.33 0.88 1.33 1.62 1.70 1.66 1.14 0.88 0.70 0.95 1.28 0.95
3.31 1.47 1.89 2.27 2.21 2.41 1.52 1.23 1.01 1.29 1.43 1.55
2.5 1.7 1.4 1.4 1.3 1.5 1.3 1.4 1.4 1.4 1.4 1.6
Methanol Ethanol 1-Propanol 1-Butanol 1-Octanol 1-Dodecanol 1-Tetradecanol
6.1 5.3 7.6 11.6 14.9 19.4 35.6
1.35 1.34 1.31 1.65 1.87 1.87 1.97
1.79 1.99 2.00 2.37 3.00 3.37 3.20
1.4 1.5 1.4 1.4 1.6 1.8 1.6
a
½DMEI ¼ 1:00 mol/l, ½MAIB ¼ 5:00 102 mol/l.
H. Nakamura et al. / European Polymer Journal 38 (2002) 1995–2001
gested as a key step for limiting the molecular weight of the polymer. The radical polymerization consists of four elementary processes, namely, initiation, propagation, termination, and chain transfer. Solvent effects have been reported for all the elementary steps [16]. Among them, the effect on termination has been most often observed because termination as a diffusion-controlled process is strongly affected by the viscosity of the solvent. The propagation is the most important step since it builds up the polymer main chain structure. The solvent effect on propagation is generally small because propagation is a reaction of an electrically neutral polymer radical with a vinyl monomer. However, considerable solvent effects were observed for the propagations in some polymerization systems [8,9,16,17]. We have tried to determine the propagation rate constant (kp ) of DMEI in various solvents by means of ESR. 3.2. ESR-determination of the kp value of DMEI in various solvents by ESR Fig. 1 shows ESR spectra observed in the polymerizations of DMEI with MAIB at 50 °C in methanol as a protic solvent and in chloroform as an aprotic one, where the concentrations of DMEI and MAIB were 1.00 mol/l and 5:00 102 mol/l. Thus the polymerization system was found to show a similar five-line spectrum in both solvents. The observed spectra are ascribable to propagating poly (DMEI) radical (I) since the propagating polymer radicals of other dialkyl itaconates are reported to give rise to similar five line spectra.
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Similar spectra were also observed in the other solvents used here, viz. DMSO, toluene, chlorobenzene, benzene, ethyl acetate, acetone, DMF, THF, ACN, MEK, ethanol, 1-propanol, and 1-butanol. From the observed ESR spectra a stationary state with respect to the propagating polymer radical was found to be reached in all these solvents. The stationary concentration ([P]) of propagating poly(DMEI) radical was determined at 50 °C in the 15 solvents using its ESR spectrum; the results are shown in Table 2. From Rp above obtained and [P ], the kp values were estimated according to Eq. (1) and are also listed in Table 2; results for benzene and MEK were reported previously [7,12]. Rp ¼ kp ½P ½DMEI
ð1Þ
The kp value is considerably dependent on the kind of solvent used. For the alcoholic solvents, the higher kp is observed in the alcohol with the longer alkyl chain. The highest value (4.5 l/mol s) in toluene is 5.6 times the lowest (0.8 l/mol s) in chloroform. The solvent effects on the propagation in radical polymerization have been explained in terms of the charge transfer from the monomer to the propagating polymer radical in the transition state [18,19], donor–acceptor interaction between the propagating polymer radical and the solvent Table 2 The propagating polymer radical concentration ([P ]) and propagation rate constant (kp ) in the polymerization of DMEI with MAIB at 50 °C in various solventsa
Fig. 1. ESR spectra observed in the polymerization of DMEI with MAIB at 50 °C in methanol and in chloroform: ½DMEI ¼ 1:00 mol/l, ½MAIB ¼ 5:00 102 mol/l.
Solvent
½P 106 (mol/l)
kp (l/mol s)
DMSO Toluene Chlorobenzene Benzene Ethyl acetate Acetone DMF THF ACN MEK Chloroform Methanol Ethanol 1-Propanol 1-Butanol
7.4 2.0 3.6 2.1 2.2 2.0 3.6 1.7 3.3 2.5 3.7 4.0 2.3 2.5 2.8
2.9 4.5 2.5 3.2b 2.8 2.0 1.6 3.3 1.3 1.7b 0.8 1.5 2.3 3.0 4.0
a b
½DMEI ¼ 1:00 mol/l, ½MAIB ¼ 5:00 102 mol/l. Previously reported [7,12].
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[16,17,20], preferential solvation of the polymer radical by the monomer [16,21], and so on. However, the solvent effect for propagation in the polymerizations of DBI and DEHI as dialkyl itaconates was explicable on the basis of the interaction of propagating polymer chain with the solvent [8,9]. A similar explanation was also given for lowering of kp for methyl methacrylate in fluid CO2 compared to that in bulk [22]. The solubility parameter (SP) may be usable as a rough guide of such interaction. A linear relationship was observed between log kp of DEHI and SP of the solvent. When plotted against SP, log kp of DBI showed a maximum at about 8.5 (cal/cm3 Þ0:5 of SP, which corresponds well with 8.9 as calculated SP for poly(DBI). Fig. 2 presents the plot of ln kp of DMEI against SP. A maximum of kp is apparent at a SP value of about 10 which is considerably larger than 8.5 of SP for kp of DBI. This conforms well to the fact that poly(DMEI) is more polar than poly(DBI). Thus, a better solvent for poly(DMEI) gives a larger kp . The polymer chains are extended in good solvents, while they are contracted in poor solvents. The radical center of the propagating polymer chain are more crowded with the two oligo (ethylene oxide) side chains in poorer solvents. Such crowding hinders propagation. In order to clarify the effect of a large number of solvents on the radical copolymerizations, the linear solvation energy relationship proposed by Taft and coworkers (Eq. (2)) [23] was successfully applied to some copolymerization systems [24–27]. Y ¼ Yo þ sðp þ ddÞ þ aa þ bb
ð2Þ
Fig. 3. Correlation between the propagation rate constant (kp ) and the solvatochromic parameters: 1: toluene, 2: 1-butanol, 3: THF, 4: benzene, 5: 1-propanol, 6: DMSO, 7: acetone, 8: ethyl acetate, 9: chlorobenzene, 10: ethanol, 11: MEK, 12: DMF, 13: methanol, 14: ACN and 15: chloroform.
where Yo is the value of the given property Y in the standard solvent. p (polarity), d (polarizability), a (hydrogen-bond donor acidity), and b (hydrogen-bond acceptor basicity) are refered to as the solvatochromic parameters, and s, d, a, and b as the solvatochromic coefficients. The kp values above obtained were analyzed according to Eq. (2). As shown in Fig. 3, the following regression relationship (Eq. (3)) was obtained with a fairly high correlation coefficient (R): ln kp ¼ 0:802 1:813ðp 0:680dÞ 0:841a þ 2:328b; R ¼ 0:908 ð3Þ
Fig. 2. Relationship between the propagation rate constant (kp ) and the solubility parameter (d): 1: toluene, 2: THF, 3: benzene, 4: ethyl acetate, 5: acetone, 6: chlorobenzene, 7: MEK, 8: chloroform, 9: 1-butanol, 10: 1-propanol, 11: DMSO, 12: ethanol, 13: DMF, 14: ACN and 15: methanol.
Thus, kp is found to depend largely on polarity and hydrogen-bond acceptor basicity of the solvent although it also considerably depends on other factors. An increase in the polarity causes a decrease in kp . The decrease in kp can be compensated by an increase by the polarizability in aromatic solvents. An increase in hydrogen-bond acceptor basicity results in a large increase in kp although DMEI does not carry any hydrogen available for hydrogen bonding. This might come from a similarity in the structures of solvent and DMEI bearing many ether linkages as hydrogen-bond acceptor. Poly(DMEI) radical chain extends in a solvent similar to DMEI in structure, leading to acceleration of propagation. Specific solvation like hydrogen-bonding seems to retard the propagation. The kp value in methanol is much lower than those in higher alcohols. Mixing of chloroform with ether was reported to be an exothermic process, which is explained in terms of interaction of hydrogen of chloroform with the ether oxygen [28–30].
H. Nakamura et al. / European Polymer Journal 38 (2002) 1995–2001
DMEI was also observed to evolve heat when mixed with chloroform, indicating a similar specific interaction between DMEI with chloroform. This may be responsible for the lowest kp value in chloroform. 3.3. Solvent effect on copolymerization of DMEI with St Copolymerizations of DMEI (M1 ) and St (M2 ) with MAIB were conducted at 50 °C in various solvents, where the total monomer and the initiator concentrations were 1.00 and 5:00 102 mol/l, respectively. The nine solvents used were benzene, MEK, DMF, DMSO, THF, chloroform, ACN, methanol, and l-butanol. The copolymerization proceeded homogeneously in all the solvents. When the highest St composition (87.5 mol%) in feed was used in methanol, the obtained copolymerization mixture became turbid when cooled to near 50 °C although it was homogeneous at 50 °C, the polymerization temperature. The copolymer yields were controlled to be less than 10%. The polymer compositions were determined from the carbon contents by elemental analysis and are summarized in Table 3; results for benzene and MEK were reported previously [7,12]. The monomer reactivity ratios (r1 and r2 ) were estimated according to the curve-fitting method based on a nonlinear least-squares procedure [31], for which the copolymer composition curves were prepared using the data in Table 3. The results are listed in Table 4. A considerably large solvent effect was observed for the present copolymerization. The highest r1 value (0.46) in 1-butanol is about six times the lowest (0.08) in methanol. The r2 value falls in the range of 0.20 (DMSO) and 0.52 (benzene). On the basis of the terminal model, the copolymerization involves four kinds of propagation, the rate constants of which are k11 , k12 , k21 , and k22 respectively. k11 and k22 correspond to the kp values for DMEI and St at 50 °C. The kp values for DMEI estimated above in the nine solvents were used as k11 . The kp value of 209 l/mol s for St was taken as k22 in all the solvents used [32] be-
1999
Table 4 Copolymerization of DMEI (M1 ) and St (M2 ) at 50 °C in various solventsa Solvent
r1
r2
k11
k12
k21
k22
Benzene MEK DMF Methanol THF DMSO Chloroform ACN 1-Butanol
0.19 0.31 0.13 0.08 0.19 0.37 0.30 0.40 0.46
0.52 0.31 0.38 0.30 0.42 0.20 0.52 0.37 0.29
3.2 1.7 1.6 1.5 3.3 2.9 0.76 1.3 4.0
16.8 5.5 12.3 18.8 17.4 7.8 2.5 3.3 8.7
402 674 550 697 498 1045 402 565 721
209 209 209 209 209 209 209 209 209
a
½M1 þ ½M2 ¼ 1:00 mol/l, ½MAIB ¼ 5:00 102 mol/l.
cause kp for St was reported to be almost insensitive to the kind of solvent [33] although it was recently observed to be somewhat higher in benzyl alcohol (153 l/mol s, 30 °C) than in bulk (107 l/mol s, 30 °C) [17]. k12 and k21 were calculated from k12 ¼ k11 =r1 and k21 ¼ k22 =r2 , respectively and are also shown in Table 4. The highest k12 value (18.8 l/mol s) in methanol is 7.5 times the lowest (2.5 l/mol s) in chloroform. Eq. (2) was applied to the k12 value, giving Eq. (4). The correlation coefficient (R ¼ 0:709) was not high. Its plot is presented in Fig. 4. Thus, the solvatochromic coefficients for k12 tend to be similar to those for kp , i.e., k11 , although the acceleration effect due to polarizability is higher for the former. The similarity comes probably from the fact that both propagations involve the poly(DMEI) radical. ln k12 ¼ 1:603 1:290ðp 1:166dÞ 0:113a þ 2:450b; R ¼ 0:709 ð4Þ The highest k21 value (1045 l/mol s) in DMSO is 2.6 times the lowest (402 l/mol s) in benzene. This value was also analyzed by Eq. (2) and Eq. (5) was obtained with R ¼ 0:875. Fig. 5 shows its plot. In contrast to the cases of k11 (kp ) and k12 , increase in the polarity causes an increase in k21 and the polarizability hardly affects k21 . Thus, the reaction of poly(St) radical with DMEI is
Table 3 Monomer–copolymer composition for the copolymerization of DMEI (M1 ) and St (M2 ) with MAIB at 50 °C in various solventsa M1 content in feed (mol%)
Benzene
MEK
DMF
Methanol
THF
DMSO
Chloroform
ACN
1-Butanol
12.5 25.0 37.5 50.0 62.5 75.0 87.5
17.0 30.0 37.4 43.6 50.7 56.6 69.0
– 37.1 45.8 53.7 55.1 62.1 79.5
21.1 31.0 41.4 44.8 50.4 56.4 62.4
23.5 34.7 39.5 45.7 49.1 53.8 60.0
– 30.9 40.2 46.9 49.8 58.9 67.4
26.2 47.6 49.7 48.2 59.2 64.5 76.3
19.2 31.1 38.1 45.1 52.7 61.3 77.0
22.4 34.6 44.4 50.0 57.3 64.6 80.6
26.5 37.2 47.4 52.2 60.7 70.6 76.9
a
M1 content in the copolymer (mol%)
½M1 þ ½M2 ¼ 1:00 mol/l, ½MAIB ¼ 5:00 102 mol/l.
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H. Nakamura et al. / European Polymer Journal 38 (2002) 1995–2001
Fig. 4. Correlation between the k12 value and the solvatochromic parameters: 1: THF, 2: benzene, 3: methanol, 4: DMF, 5: DMSO, 6: MEK, 7: ACN, 8: 1-butanol and 9: chloroform.
highest Rp at 50 °C in 1-tetradecanol is 13 times the lowest in chloroform. On the other hand, the molecular weight of the resulting poly(DMEI)s is almost insensitive to the solvent used. The ESR-determined kp value at 50 °C shows a noticeable solvent effect; the highest value (4.5 l/mol s) in toluene is 5.6 times the lowest (0.8 l/mol s) in chloroform. Analysis according to the linear solvation energy relationship proposed by Taft and co-workers suggests that the propagation is retarded in polar solvents and accelerated in solvents having structures similar to DMEI. Copolymerization of DMEI(M1 ) and St(M2 ) is also largely influenced by the kind of solvent used. The highest r1 value (0.46) in l-butanol is about six times the lowest (0.08) in methanol. The r2 value is in the range 0.20 (DMSO) to 0.52 (benzene). It is suggested that the reactivity of poly (DMEI) radical is decreased and that of DMEI monomer is increased in polar solvents.
References
Fig. 5. Correlation between the k21 value and the solvatochromic parameters: same symbols as in Fig. 4.
accelerated in polar solvents, indicating that the polar solvents enhance the reactivity of DMEI monomer. The monomers may associate in the polar solvents, causing a bootstrap effect [20]. ln k21 ¼ 5:423 þ 0:870ðp 0:041dÞ þ 0:241a þ 0:623b; R ¼ 0:875 ð5Þ
4. Conclusions The polymerization rate (Rp ) of DMEI with MAIB strongly depends on the kind of solvent used. The
[1] Sato T, Inui S, Tanaka H, Ota T, Kamachi M, Tanaka K. J Polym Sci Polym Chem Ed 1987;25:637. [2] Sato T, Morino K, Tanaka H, Ota T. Makromol Chem 1987;188:2951. [3] Otsu T, Yamagishi K, Yoshioka M. Macromolecules 1992; 25:2713. [4] Otsu T, Yamagishi K, Matsumoto A, Yoshioka M, Watanabe H. Macromolecules 1993;26:3026. [5] Sato T, Hirose M, Tanaka H, Uchiumi N, Matsumoto M. Eur Polym J 1994;30:347. [6] Sato T, Hirose Y, Seno M, Tanaka H. J Polym Sci A: Polym Chem 1995;33:717. [7] Nakamura H, Seno M, Tanaka H, Sato T. Colloid Polym Sci 1995;273:122. [8] Sato T, Morita N, Tanaka H, Ota T. J Polym Sci A 1989;27:2497. [9] Sato T, Shimizu T, Seno M, Tanaka H, Ota T. Makromol Chem 1992;193:1439. [10] Sato T, Nakamura H, Tanaka H, Ota T. Makromol Chem 1991;192:2659. [11] Nakamura H, Seno M, Tanaka H, Sato T. Makromol Chem 1993;194:1773. [12] Nakamura H, Seno M, Sato T. J Polym Sci A: Polym Chem 1997;35:153. [13] Sato T, Takahashi K, Tanaka H, Ota T. Macromolecules 1991;24:2330. [14] Sato T, Kawasaki S, Seno M, Tanaka H, Kato T. Makoromol Chem 1993;194:2247. [15] Sato T, Takahashi Y, Seno M, Nakamura H, Tanaka H, Ota T. Makromol Chem 1991;192:2909. [16] Kamachi M. Adv Polym Sci 1980;38:55. [17] O0 Driscoll F, Monteiro MJ, Klumperman B. J Polym Sci A: Polym Chem 1997;35:515. [18] Kinoshita M, Imoto M. Kogyo Kagaku Zasshi 1965;68:2454. [19] Imoto M, Ouchi T. Nippon Kagaku Kaishi 1987:894.
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[27] Geogiev GS, Dakova IG. Macromol Chem Phys 1994; 195:1695. [28] Beath LA, Williamson AG. J Chem Thermodynamics 1969;1:51. [29] Handa YP, Mattingley BI, Fenby DV. J Chem Soc Faraday Trans 1 1976;72:1355. [30] Marvel CS, Copley MJ, Ginsberg E. J Am Chem Soc 1940;52:3109. [31] Yamada B, Itahashi M, Otsu T. J Polym Sci: Polym Chem Ed 1978;16:1719. [32] Davis TP, O0 Driscoll KF, Piton MC, Winnik MA. Macromolecules 1990;23:2113. [33] Yamamoto T, Yamamoto T, Mito A, Hirota M. Nippon Kagaku Kaishi 1978:1561.
Solvent effect on radical homo- and copolymerization of di-2-[2-(2-methoxyethoxy)ethoxy]ethyl itaconate H. Nakamura, M. Seno, H. Naganuma, T. Sato
*
Faculty of Engineering, Department of Chemical Science and Technology, Tokushima University, Minamijosanjima 2-1, Tokushima 770-8506, Japan Received 16 November 2001; received in revised form 4 February 2002; accepted 7 February 2002
Abstract Solvent effect on homo- and copolymerization of di-2-[2-(methoxyethoxy)ethoxy]ethyl itaconate (DMEI) was studied at 50 °C using dimethyl 2,20 -azobisisobutyrate as radical initiator. The polymerization rate (Rp ) highly depended on the kind of solvent; 19 solvents were used. The highest Rp (in 1-tetradecanol) is 13 times the smallest (in chloroform). On the other hand, the solvents did not exert as great an effect on the molecular weight of the resulting polymers. The propagation rate constant (kp ) was determined in 15 different solvents by means of ESR spectroscopy. The highest kp (4.5 l/mol s in toluene) is 5.6 times the lowest (0.8 l/mol s in chloroform). A noticeable solvent effect was also observed in the copolymerization of DMEI (M1 ) and styrene (M2 ), where nine solvents were used. The highest r1 (0.46 in 1-butanol) is about 6 times the lowest (0.08 in methanol). The r2 value falls in the range of 0.2 (dimethyl sulfoxide) and 0.52 (benzene). The solvent effects thus observed were analyzed according to the linear solvation energy relationship proposed by Taft and co workers. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Itaconate; Radical polymerization; Solvent effect; ESR spectrum; Rate constant of propagation; Monomer reactivity ratio
1. Introduction Dialkyl itaconates are readily polymerized by radical initiators in spite of the severe steric requirements of their bulky a-substituents. The homogeneous polymerization systems involve ESR-observable propagating polymer radicals under actual polymerization conditions because the termination reactions are much suppressed by the substituents [1–7]. Therefore, the polymerization systems have often been used for ESR study of solvent effect [8,9], salt effect [10–12], penultimate effect [13,14], and substituent effect [3,4,15] in homogeneous radical polymerization.
Previously we found that the radical polymerization rates of dialkyl itaconates, such as di-n-butyl itaconate (DBI) and di-2-ethylhexyl itaconate (DEHI), highly depend on the kind of solvent used, and explained the large solvent effect in terms of the solvent affinity for the propagating polymer radicals [8,9]. Recently, we have observed that di-2-[2-(2-methoxyethoxy)ethoxy]ethyl itaconate (DMEI) carrying two hydrophilic oligomeric ethylene oxide side chains also experiences a significant solvent effect in the radical polymerization and copolymerization, and have studied kinetically the solvent effect by means of ESR.
*
Corresponding author. Tel.: +81-88-656-7402; fax: +81-88655-7025. E-mail address: [email protected] (T. Sato).
This paper describes the results on solvent effect during the radical polymerization and copolymerization
0014-3057/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 0 2 ) 0 0 0 7 9 - 4
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H. Nakamura et al. / European Polymer Journal 38 (2002) 1995–2001
of DMEI on the basis of elementary reaction analysis by ESR.
2. Experimental 2.1. Materials DMEI was prepared by acid-catalyzed esterification of itaconic acid with 2-[2-(2-methoxyethoxy)ethoxy]ethanol using p-toluenesulfonic acid; it was purified by column chromatography using silica gel as packing and ethyl acetate as developing solvent as described previously [7]. Dimethyl 2,20 -azobisisobutyrate (MAIB) as initiator was recrystallized from methanol. Solvents were used after distillation. Commercial styrene (St) was freed from inhibitor by washing with 5% NaOH aqueous solution, dried over anhydrous Na2 SO4 , and distilled. 2.2. Measurements Gel permeation chromatography (GPC) was performed at 38 °C on a Toso HLC-802A chromotograph using tetrahydrofuran (THF) as eluent. From the GPC results, the number-average (M n ) and weight-average (M w ) molecular weights were estimated after calibration with poly(St) standards. 1 H-NMR spectra were recorded with a Hitachi R-24b (60 MHz) spectrometer. ESR spectra of the polymerization mixtures in degassed and sealed ESR tube were obtained using a JEOL JESFE2XG spectrometer at X-band (9.5 GHz) with a transverse electric wave mode cavity. The concentration of polymer radicals was determined by computer double integration of the first derivative ESR spectrum. For this, a solution of 2,2,6,6-tetramethylpiperidin-l-oxyl radical(TEMPO) in benzene was used as standard, since TEMPO was unstable in the present polymerization system. A MgO marker was used for the calibration of the radical concentration in various media. 2.3. Polymerization Polymerization and copolymerization of DMEI were carried out in degassed and sealed glass tubes with shaking at 50 °C. The resulting poly (DMEI) which contained a small amount (10%) of the monomer was isolated by pouring the polymerization mixture into a large excess of diethyl ether-cyclohexane (5:3(v/v)) mixture. Then, the conversion was corrected by GPC using a calibration curve for the relative weight between DMEI and its polymer. The conversion in the polymerization in dimethyl sulfoxide (DMSO) was determined by following the decrease in the vinyl proton signals of monomer by NMR, where the o-methylene protons of benzyl acetate were used as internal standard.
Copolymers of DMEI and St were also isolated using the diethyl ether-cyclohexane mixture as precipitant. 3. Results and discussion 3.1. Polymerization of DMEI with MAIB in various solvents DMEI was polymerized with MAIB at 50 °C in various solvents, where the concentrations of DMEI and MAIB were 1.00 mol/l and 5:00 102 mol/l, respectively. The 19 solvents used were benzene, toluene, chlorobenzene, ethyl acetate, acetone, methyl ethyl ketone (MEK), THF, chloroform, acetonitrile (ACN), diethylene glycol dimethyl ether (diglyme), N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), methanol, ethanol, 1-propanol, 1-butanol, 1-octanol, 1dodecanol, 1-tetradecanol. The polymerization proceeded homogeneously in these solvents. The polymer yield increased linearly with time without any induction period. From the time– conversion plots, the initial polymerization rate (Rp ) was estimated in each solvent. Table 1 summarizes the results, together with the molecular weights of the polymers. Rp was found to depend highly on the kind of solvent used. The highest Rp observed in 1-tetradecanol is 13 times the smallest in chloroform. On the other hand, the solvents did not exert as large an effect on the molecular weight (M n ¼ 0:7 103 –2:0 103 ) of poly(DMEI)s formed. Chain transfer to monomer is sugTable 1 Solvent effect on the polymerization of DMEI with MAIB at 50 °Ca Solvent
Rp 106 (mol/l)
M n =104
M w =104
M w =M n
Diglyme DMSO Toluene Chlorobenzene Benzene Ethyl acetate Acetone DMF THF ACN MEK Chloroform
24.1 21.5 9.0 8.9 6.8 6.2 5.8 5.8 5.6 4.4 4.2 2.8
1.33 0.88 1.33 1.62 1.70 1.66 1.14 0.88 0.70 0.95 1.28 0.95
3.31 1.47 1.89 2.27 2.21 2.41 1.52 1.23 1.01 1.29 1.43 1.55
2.5 1.7 1.4 1.4 1.3 1.5 1.3 1.4 1.4 1.4 1.4 1.6
Methanol Ethanol 1-Propanol 1-Butanol 1-Octanol 1-Dodecanol 1-Tetradecanol
6.1 5.3 7.6 11.6 14.9 19.4 35.6
1.35 1.34 1.31 1.65 1.87 1.87 1.97
1.79 1.99 2.00 2.37 3.00 3.37 3.20
1.4 1.5 1.4 1.4 1.6 1.8 1.6
a
½DMEI ¼ 1:00 mol/l, ½MAIB ¼ 5:00 102 mol/l.
H. Nakamura et al. / European Polymer Journal 38 (2002) 1995–2001
gested as a key step for limiting the molecular weight of the polymer. The radical polymerization consists of four elementary processes, namely, initiation, propagation, termination, and chain transfer. Solvent effects have been reported for all the elementary steps [16]. Among them, the effect on termination has been most often observed because termination as a diffusion-controlled process is strongly affected by the viscosity of the solvent. The propagation is the most important step since it builds up the polymer main chain structure. The solvent effect on propagation is generally small because propagation is a reaction of an electrically neutral polymer radical with a vinyl monomer. However, considerable solvent effects were observed for the propagations in some polymerization systems [8,9,16,17]. We have tried to determine the propagation rate constant (kp ) of DMEI in various solvents by means of ESR. 3.2. ESR-determination of the kp value of DMEI in various solvents by ESR Fig. 1 shows ESR spectra observed in the polymerizations of DMEI with MAIB at 50 °C in methanol as a protic solvent and in chloroform as an aprotic one, where the concentrations of DMEI and MAIB were 1.00 mol/l and 5:00 102 mol/l. Thus the polymerization system was found to show a similar five-line spectrum in both solvents. The observed spectra are ascribable to propagating poly (DMEI) radical (I) since the propagating polymer radicals of other dialkyl itaconates are reported to give rise to similar five line spectra.
1997
Similar spectra were also observed in the other solvents used here, viz. DMSO, toluene, chlorobenzene, benzene, ethyl acetate, acetone, DMF, THF, ACN, MEK, ethanol, 1-propanol, and 1-butanol. From the observed ESR spectra a stationary state with respect to the propagating polymer radical was found to be reached in all these solvents. The stationary concentration ([P]) of propagating poly(DMEI) radical was determined at 50 °C in the 15 solvents using its ESR spectrum; the results are shown in Table 2. From Rp above obtained and [P ], the kp values were estimated according to Eq. (1) and are also listed in Table 2; results for benzene and MEK were reported previously [7,12]. Rp ¼ kp ½P ½DMEI
ð1Þ
The kp value is considerably dependent on the kind of solvent used. For the alcoholic solvents, the higher kp is observed in the alcohol with the longer alkyl chain. The highest value (4.5 l/mol s) in toluene is 5.6 times the lowest (0.8 l/mol s) in chloroform. The solvent effects on the propagation in radical polymerization have been explained in terms of the charge transfer from the monomer to the propagating polymer radical in the transition state [18,19], donor–acceptor interaction between the propagating polymer radical and the solvent Table 2 The propagating polymer radical concentration ([P ]) and propagation rate constant (kp ) in the polymerization of DMEI with MAIB at 50 °C in various solventsa
Fig. 1. ESR spectra observed in the polymerization of DMEI with MAIB at 50 °C in methanol and in chloroform: ½DMEI ¼ 1:00 mol/l, ½MAIB ¼ 5:00 102 mol/l.
Solvent
½P 106 (mol/l)
kp (l/mol s)
DMSO Toluene Chlorobenzene Benzene Ethyl acetate Acetone DMF THF ACN MEK Chloroform Methanol Ethanol 1-Propanol 1-Butanol
7.4 2.0 3.6 2.1 2.2 2.0 3.6 1.7 3.3 2.5 3.7 4.0 2.3 2.5 2.8
2.9 4.5 2.5 3.2b 2.8 2.0 1.6 3.3 1.3 1.7b 0.8 1.5 2.3 3.0 4.0
a b
½DMEI ¼ 1:00 mol/l, ½MAIB ¼ 5:00 102 mol/l. Previously reported [7,12].
1998
H. Nakamura et al. / European Polymer Journal 38 (2002) 1995–2001
[16,17,20], preferential solvation of the polymer radical by the monomer [16,21], and so on. However, the solvent effect for propagation in the polymerizations of DBI and DEHI as dialkyl itaconates was explicable on the basis of the interaction of propagating polymer chain with the solvent [8,9]. A similar explanation was also given for lowering of kp for methyl methacrylate in fluid CO2 compared to that in bulk [22]. The solubility parameter (SP) may be usable as a rough guide of such interaction. A linear relationship was observed between log kp of DEHI and SP of the solvent. When plotted against SP, log kp of DBI showed a maximum at about 8.5 (cal/cm3 Þ0:5 of SP, which corresponds well with 8.9 as calculated SP for poly(DBI). Fig. 2 presents the plot of ln kp of DMEI against SP. A maximum of kp is apparent at a SP value of about 10 which is considerably larger than 8.5 of SP for kp of DBI. This conforms well to the fact that poly(DMEI) is more polar than poly(DBI). Thus, a better solvent for poly(DMEI) gives a larger kp . The polymer chains are extended in good solvents, while they are contracted in poor solvents. The radical center of the propagating polymer chain are more crowded with the two oligo (ethylene oxide) side chains in poorer solvents. Such crowding hinders propagation. In order to clarify the effect of a large number of solvents on the radical copolymerizations, the linear solvation energy relationship proposed by Taft and coworkers (Eq. (2)) [23] was successfully applied to some copolymerization systems [24–27]. Y ¼ Yo þ sðp þ ddÞ þ aa þ bb
ð2Þ
Fig. 3. Correlation between the propagation rate constant (kp ) and the solvatochromic parameters: 1: toluene, 2: 1-butanol, 3: THF, 4: benzene, 5: 1-propanol, 6: DMSO, 7: acetone, 8: ethyl acetate, 9: chlorobenzene, 10: ethanol, 11: MEK, 12: DMF, 13: methanol, 14: ACN and 15: chloroform.
where Yo is the value of the given property Y in the standard solvent. p (polarity), d (polarizability), a (hydrogen-bond donor acidity), and b (hydrogen-bond acceptor basicity) are refered to as the solvatochromic parameters, and s, d, a, and b as the solvatochromic coefficients. The kp values above obtained were analyzed according to Eq. (2). As shown in Fig. 3, the following regression relationship (Eq. (3)) was obtained with a fairly high correlation coefficient (R): ln kp ¼ 0:802 1:813ðp 0:680dÞ 0:841a þ 2:328b; R ¼ 0:908 ð3Þ
Fig. 2. Relationship between the propagation rate constant (kp ) and the solubility parameter (d): 1: toluene, 2: THF, 3: benzene, 4: ethyl acetate, 5: acetone, 6: chlorobenzene, 7: MEK, 8: chloroform, 9: 1-butanol, 10: 1-propanol, 11: DMSO, 12: ethanol, 13: DMF, 14: ACN and 15: methanol.
Thus, kp is found to depend largely on polarity and hydrogen-bond acceptor basicity of the solvent although it also considerably depends on other factors. An increase in the polarity causes a decrease in kp . The decrease in kp can be compensated by an increase by the polarizability in aromatic solvents. An increase in hydrogen-bond acceptor basicity results in a large increase in kp although DMEI does not carry any hydrogen available for hydrogen bonding. This might come from a similarity in the structures of solvent and DMEI bearing many ether linkages as hydrogen-bond acceptor. Poly(DMEI) radical chain extends in a solvent similar to DMEI in structure, leading to acceleration of propagation. Specific solvation like hydrogen-bonding seems to retard the propagation. The kp value in methanol is much lower than those in higher alcohols. Mixing of chloroform with ether was reported to be an exothermic process, which is explained in terms of interaction of hydrogen of chloroform with the ether oxygen [28–30].
H. Nakamura et al. / European Polymer Journal 38 (2002) 1995–2001
DMEI was also observed to evolve heat when mixed with chloroform, indicating a similar specific interaction between DMEI with chloroform. This may be responsible for the lowest kp value in chloroform. 3.3. Solvent effect on copolymerization of DMEI with St Copolymerizations of DMEI (M1 ) and St (M2 ) with MAIB were conducted at 50 °C in various solvents, where the total monomer and the initiator concentrations were 1.00 and 5:00 102 mol/l, respectively. The nine solvents used were benzene, MEK, DMF, DMSO, THF, chloroform, ACN, methanol, and l-butanol. The copolymerization proceeded homogeneously in all the solvents. When the highest St composition (87.5 mol%) in feed was used in methanol, the obtained copolymerization mixture became turbid when cooled to near 50 °C although it was homogeneous at 50 °C, the polymerization temperature. The copolymer yields were controlled to be less than 10%. The polymer compositions were determined from the carbon contents by elemental analysis and are summarized in Table 3; results for benzene and MEK were reported previously [7,12]. The monomer reactivity ratios (r1 and r2 ) were estimated according to the curve-fitting method based on a nonlinear least-squares procedure [31], for which the copolymer composition curves were prepared using the data in Table 3. The results are listed in Table 4. A considerably large solvent effect was observed for the present copolymerization. The highest r1 value (0.46) in 1-butanol is about six times the lowest (0.08) in methanol. The r2 value falls in the range of 0.20 (DMSO) and 0.52 (benzene). On the basis of the terminal model, the copolymerization involves four kinds of propagation, the rate constants of which are k11 , k12 , k21 , and k22 respectively. k11 and k22 correspond to the kp values for DMEI and St at 50 °C. The kp values for DMEI estimated above in the nine solvents were used as k11 . The kp value of 209 l/mol s for St was taken as k22 in all the solvents used [32] be-
1999
Table 4 Copolymerization of DMEI (M1 ) and St (M2 ) at 50 °C in various solventsa Solvent
r1
r2
k11
k12
k21
k22
Benzene MEK DMF Methanol THF DMSO Chloroform ACN 1-Butanol
0.19 0.31 0.13 0.08 0.19 0.37 0.30 0.40 0.46
0.52 0.31 0.38 0.30 0.42 0.20 0.52 0.37 0.29
3.2 1.7 1.6 1.5 3.3 2.9 0.76 1.3 4.0
16.8 5.5 12.3 18.8 17.4 7.8 2.5 3.3 8.7
402 674 550 697 498 1045 402 565 721
209 209 209 209 209 209 209 209 209
a
½M1 þ ½M2 ¼ 1:00 mol/l, ½MAIB ¼ 5:00 102 mol/l.
cause kp for St was reported to be almost insensitive to the kind of solvent [33] although it was recently observed to be somewhat higher in benzyl alcohol (153 l/mol s, 30 °C) than in bulk (107 l/mol s, 30 °C) [17]. k12 and k21 were calculated from k12 ¼ k11 =r1 and k21 ¼ k22 =r2 , respectively and are also shown in Table 4. The highest k12 value (18.8 l/mol s) in methanol is 7.5 times the lowest (2.5 l/mol s) in chloroform. Eq. (2) was applied to the k12 value, giving Eq. (4). The correlation coefficient (R ¼ 0:709) was not high. Its plot is presented in Fig. 4. Thus, the solvatochromic coefficients for k12 tend to be similar to those for kp , i.e., k11 , although the acceleration effect due to polarizability is higher for the former. The similarity comes probably from the fact that both propagations involve the poly(DMEI) radical. ln k12 ¼ 1:603 1:290ðp 1:166dÞ 0:113a þ 2:450b; R ¼ 0:709 ð4Þ The highest k21 value (1045 l/mol s) in DMSO is 2.6 times the lowest (402 l/mol s) in benzene. This value was also analyzed by Eq. (2) and Eq. (5) was obtained with R ¼ 0:875. Fig. 5 shows its plot. In contrast to the cases of k11 (kp ) and k12 , increase in the polarity causes an increase in k21 and the polarizability hardly affects k21 . Thus, the reaction of poly(St) radical with DMEI is
Table 3 Monomer–copolymer composition for the copolymerization of DMEI (M1 ) and St (M2 ) with MAIB at 50 °C in various solventsa M1 content in feed (mol%)
Benzene
MEK
DMF
Methanol
THF
DMSO
Chloroform
ACN
1-Butanol
12.5 25.0 37.5 50.0 62.5 75.0 87.5
17.0 30.0 37.4 43.6 50.7 56.6 69.0
– 37.1 45.8 53.7 55.1 62.1 79.5
21.1 31.0 41.4 44.8 50.4 56.4 62.4
23.5 34.7 39.5 45.7 49.1 53.8 60.0
– 30.9 40.2 46.9 49.8 58.9 67.4
26.2 47.6 49.7 48.2 59.2 64.5 76.3
19.2 31.1 38.1 45.1 52.7 61.3 77.0
22.4 34.6 44.4 50.0 57.3 64.6 80.6
26.5 37.2 47.4 52.2 60.7 70.6 76.9
a
M1 content in the copolymer (mol%)
½M1 þ ½M2 ¼ 1:00 mol/l, ½MAIB ¼ 5:00 102 mol/l.
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H. Nakamura et al. / European Polymer Journal 38 (2002) 1995–2001
Fig. 4. Correlation between the k12 value and the solvatochromic parameters: 1: THF, 2: benzene, 3: methanol, 4: DMF, 5: DMSO, 6: MEK, 7: ACN, 8: 1-butanol and 9: chloroform.
highest Rp at 50 °C in 1-tetradecanol is 13 times the lowest in chloroform. On the other hand, the molecular weight of the resulting poly(DMEI)s is almost insensitive to the solvent used. The ESR-determined kp value at 50 °C shows a noticeable solvent effect; the highest value (4.5 l/mol s) in toluene is 5.6 times the lowest (0.8 l/mol s) in chloroform. Analysis according to the linear solvation energy relationship proposed by Taft and co-workers suggests that the propagation is retarded in polar solvents and accelerated in solvents having structures similar to DMEI. Copolymerization of DMEI(M1 ) and St(M2 ) is also largely influenced by the kind of solvent used. The highest r1 value (0.46) in l-butanol is about six times the lowest (0.08) in methanol. The r2 value is in the range 0.20 (DMSO) to 0.52 (benzene). It is suggested that the reactivity of poly (DMEI) radical is decreased and that of DMEI monomer is increased in polar solvents.
References
Fig. 5. Correlation between the k21 value and the solvatochromic parameters: same symbols as in Fig. 4.
accelerated in polar solvents, indicating that the polar solvents enhance the reactivity of DMEI monomer. The monomers may associate in the polar solvents, causing a bootstrap effect [20]. ln k21 ¼ 5:423 þ 0:870ðp 0:041dÞ þ 0:241a þ 0:623b; R ¼ 0:875 ð5Þ
4. Conclusions The polymerization rate (Rp ) of DMEI with MAIB strongly depends on the kind of solvent used. The
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